Field of the Invention
The present invention relates to step-up/down switching regulators.
Description of Related Art
In start-stop (reduced-idling) vehicles, in which the engine is restarted many times, during a period when the engine is stopped temporarily, the electric power of the battery keeps being consumed by vehicle-mounted appliances such as audiovisual and air-conditioning equipment. Such vehicles thus suffer a severer drop then ever in the battery voltage at the time of cranking (when the engine is restarted). By use of a step-up/down switching regulator, which holds the output voltage on occurrence of a drop in the input voltage (battery voltage), it is possible to let vehicle-mounted appliances operate normally even on occurrence of a sharp drop in the battery voltage at the time of cranking.
Accordingly, needs for step-up/down switching regulators are increasing in the market of vehicle-mounted appliances.
A description will now be given of the configuration and operation of a common step-up/down switching regulator. FIG. 38 is a diagram showing the configuration of a common step-up/down switching regulator.
The switching regulator shown in FIG. 38 includes MOS transistors Q11 and Q12 as step-down switches, an inductor L11, MOS transistors Q13 and Q14 as step-up switches, an output capacitor C11, resistors R11 to R14, and a controller CNT11.
The controller CNT11 monitors the output voltage VOUT on the basis of the output of a voltage division circuit constituted by the resistors R11 and R12, and monitors the battery voltage VBAT on the basis of the output of a voltage division circuit constituted by the resistors R13 and R14.
When the battery voltage VBAT is higher than a first predetermined value A1, the controller CNT11 selects a step-down mode (see FIG. 39). In the step-down mode, the controller CNT11 turns ON and OFF the MOS transistors Q11 and Q12 according to the output voltage VOUT, while keeping the MOS transistor Q13 OFF and the MOS transistor Q14 ON all the time. As a result, a first switching voltage VSW1, which is the voltage at the connection node between the MOS transistors Q11 and Q12, and a second switching voltage VSW2, which is the voltage at the connection node between the MOS transistors Q13 and Q14, behave as shown in FIG. 40A.
When the battery voltage VBAT is equal to or lower than the first predetermined value A1 but higher than a second predetermined value A2, the controller CNT11 selects a step-up/down mode (see FIG. 39). In the step-up/down mode, the controller CNT11 turns ON and OFF the MOS transistors Q11 and Q12 according to the output voltage VOUT, and turns ON and OFF the MOS transistors Q13 and Q14 according to the output voltage VOUT. As a result, the first switching voltage VSW1, which is the voltage at the connection node between the MOS transistors Q11 and Q12, and the second switching voltage VSW2, which is the voltage at the connection node between the MOS transistors Q13 and Q14, behave as shown in FIG. 40B.
When the battery voltage VBAT is equal to or lower than the second predetermined value A2, the controller CNT11 selects a step-up mode (see FIG. 39). In the step-up mode, the controller CNT11, while keeping the MOS transistor Q11 ON and the MOS transistor Q12 OFF all the time, turns ON and OFF the MOS transistors Q13 and Q14 according to the output voltage VOUT. As a result, the first switching voltage VSW1, which is the voltage at the connection node between the MOS transistors Q11 and Q12, and the second switching voltage VSW2, which is the voltage at the connection node between the MOS transistors Q13 and Q14, behave as shown in FIG. 40C.
In the step-up/down and step-up modes, in which the MOS transistors Q13 and Q14 perform step-up operation, the transfer function H(s) of the PWM (pulse-width modulation) modulator constituted by the MOS transistors Q13 and Q14 in the step-up/down switching regulator shown in FIG. 38 includes a term T(s) that is given by formula (1) below.
                              T          ⁡                      (            s            )                          =                  1          -                                                    α                ⁢                                                                  ⁢                L                ⁢                                                      I                    _                                    L                                                                              (                                      1                    -                    D                                    )                                ⁢                                                      V                    _                                    OUT                                                      ⁢            s                                              (        1        )                            where        ĪL represents the average current through the inductor L11;        L represents the inductance value of the inductor L11;        VOUT represents the average value of the output voltage VOUT;        D represents the ON-duty of the MOS transistor Q13; and        α represents the constant of proportion.        
The term T(s) given by formula (1) above represents right-half-plane-zero characteristics (characteristics in which a zero point exists in the right half plane). With the step-up/down switching regulator shown in FIG. 38, a satisfactory response is expected only in a range lower than the frequency f given by formula (2) below. Formula (2) below is obtained by substituting s=jω=j·2πf in formula (1) above.
                    f        =                              1                          2              ⁢              π                                ·                                                    (                                  1                  -                  D                                )                            ⁢                                                V                  _                                OUT                                                    α              ⁢                                                          ⁢              L              ⁢                                                I                  _                                L                                                                        (        2        )            
One way to improve the response is to increase the capacitance of the output capacitor C11 in the step-up/down switching regulator shown in FIG. 38. Another way is to switch to a configuration in which a step-down switching regulator portion is provided in a stage subsequently to a step-up switching regulator portion.
However, the former measure suffers from increased cost for the output capacitor C11, and the latter measure suffers from increased cost for reactors because the step-up and step-down switching regulator portions each require a separate reactor.
Japanese Patent No. 3556652 (claim 7, FIG. 11) discloses a DC-DC converter that solves the above problems. Inconveniently, however, its circuit configuration is such that the duty of a first control signal generated by a feedforward control circuit depends on the input voltage; consequently, if the duty causes linear variation in the transfer function of the DC-DC converter, it is difficult to compensate for.